The development of new, cost-competitive processes that utilize renewable resources as feedstocks is vital for a sustainable economy. These processes also represent important milestones toward the goal of reducing the United States' dependence on foreign oil. Introduction of such processes not only avoids the use of more petroleum, but also has the potential to provide substantial energy savings, and reduce greenhouse gas emissions. Although biobased synthesis of certain commercial significant compounds such as 1,3-propanediol have been reported,1 there are comparatively few reported approaches to compounds related to terephthalic acid.2 
Terephthalic acid (4-carboxybenzoic acid), is a commodity chemical produced from petroleum feedstocks. The most common synthesis pathway is the oxidation of para-xylene. Terephthalic acid and dimethyl terephthalate are employed in the preparation of polyethylene terephthalate (PET), a thermoplastic polymer used in many beverage and food containers and in fabrics, and polytrimethylene terephthalate, a material used in carpets and upholstery. Global production of terephthalic acid was near fifty million tons in 2009.
The regioselective Diels-Alder reactions of coumalic acid (1) with alpha-olefins has been reported,3 as shown in Scheme 1, wherein R is alkyl, alkoxy, aryl, aryloxy, alkenyl and the like. The process involves a Diels-Alder reaction to produce a bicyclic intermediate that can be dehydrogenated by a Pd/C catalyst with loss of carbon dioxide to form the para-substituted benzoic acid in 99% para-selectivity and in yields in the range of 60-70%.

Dimethyl terephthalate can be prepared from methyl coumalate (1) by replacing R—CH═CH2 with a captodative diene such as methyl (α-methoxy) acrylate.4 
In order for the use of the Diels-Alder reaction to prepare terephthalic acid to become industrially useful, a viable and scalable synthesis of coumalic acid is needed. The conversion of malic acid (2) into coumalic acid is well known on a laboratory scale using concentrated sulfuric acid (97-98%) as the solvent and fuming sulfuric acid, a corrosive dehydrating agent, as the reagent. The reaction is conducted at 70° C. This transformation, shown in Scheme 2, was reported by von Pechmann in 1891 and appears to be the only reported preparation.5 An Organic Synthesis article describes coumalic acid synthesis on a 100 gram scale using the von Pechmann conditions.6
Recently, Kaminski and Kirsh described a synthesis of 1 using a more concentrated solution of sulfuric acid.7 
The intermediate in this transformation is formyl acetic acid (HO2C—CH2—CHO). Two molecules of this aldehyde acid react to produce one molecule of coumalic acid. Although this reaction is suitable for a multigram laboratory scale, scaling these corrosive reaction conditions to a pilot plant scale is not feasible.
The mechanism by which malic acid is transformed into the aldehyde acid was recently studied.8 There is vigorous gas evolution at the beginning of the reaction. The gas is carbon monoxide, suggesting a direct protonation of the carboxylic acid as an early step. Interestingly, less than five percent of fumaric acid (3) is produced under these acidic conditions.
However, a need clearly exists for methods to produce coumalic acid (1) under milder conditions.